Displacement pumps generally exhibit certain characteristics which make them less suited to certain applications, although the displacement pump may be preferred in view of other of its characteristics. One such characteristic, which makes most displacement pumps difficult to use with aggressive fluids and fluids with particles mixed in or in suspension, expecially those which cause wear, is the fact that most displacement pumps include parts which are required to move in relation to each other during sealing. The situation becomes even more problematical if the fluid being pumped has a tendency to cake and to become attached to the walls of the pump. Examples of fluids of this kind which are difficult to pump are asphalt, concrete, sewage, molasses and paper pulp. Experiments have been conducted in an attempt to modify displacement pumps for pumping of this kind by fitting scraper rings or by manufacturing the component parts of the pump from a soft material such as rubber. In spite of the fact that these experiments have produced excellent pump designs in many cases, the need still remains for further types of pumps suitable for particularly difficult fluids, amongst which concrete can be mentioned, which has a viscous consistency, is extremely abrasive and is difficult to remove once it has become baked and has assumed a solid state.